Recently, with the strong demand for the deep-ultraviolet laser source (generally refers to the laser source emitting a laser with a wavelength of shorter than 200 nm) of 193 nm lithography technology, micro-nano fine laser processing, and modern instruments such as ultra-high energy resolution photoelectron spectrometer and photoelectron emission microscopy, developing an all-solid-state deep-ultraviolet laser source has become a research hotspot of the international laser scientific community. The deep-ultraviolet nonlinear optical crystal is a key element to develop the all-solid-state deep-ultraviolet laser source.
Currently, the ultraviolet and deep-ultraviolet nonlinear optical crystals used in the industries mainly include LiB3O5 (LBO), CsB3O5 (CBO), CsLiB6O10 (CLBO), BaB2O4 (BBO), and KBe2BO3F2(KBBF) crystals. LBO crystal has a wide transmission region, a good optical homogeneity, a large effective frequency multiplication factor (3KDP) and a high damage threshold (18.9 GW/cm2). However, LBO crystal is unable to achieve phase matching in the deep-ultraviolet region due to its relatively small birefringence (Δn=0.04-0.05) and the shortest second harmonic wavelength is 276 nm. Similar to LBO crystal, CBO and CLBO crystals also have limited applications in the deep-ultraviolet region due to the relatively small birefringence. Although BBO crystal has a large frequency-doubling coefficient and a large birefringence, its relatively high ultraviolet absorption cutoff edge (189 nm) and the shortest second harmonic wavelength of 204.8 nm limit its application in the deep-ultraviolet region. KBBF can output a frequency-sextupled light from a 1064 nm fundamental wavelength directly; however, it is difficult to grow a large-sized crystal due to the layered growth habit of KBBF, which limits its application to some extent. Therefore, there is an urgent need to develop a new deep-ultraviolet nonlinear optical crystal with good comprehensive performances.